The present invention relates to a spiral wound seal, and more particularly to such a seal for maintaining a pressure between chambers of a wind tunnel. A pressure seal between chambers of a wind tunnel must meet severe criteria. It must be an active (opened and closed at intervals) large diameter (up to approximately 20 feet) pressure seal for use at cryogenic temperatures. The seal must be effective at clamping pressures between 500 pounds per square inch (psi) and 2,500 psi. It must provide satisfactory sealing in a gaseous environment at temperatures between -320.degree. F. and +200.degree. F. and at pressures up to 130 psi above atmospheric. As a further requirement, because of low accessibility in a wind tunnel to the seal area, the seal must be manufactured in a relatively small size and assembled to its full size in the wind tunnel chamber.
Typically, there are three types of seal gasket configurations: (1) one-piece, (2) segmented and (3) spiral wound. Present one-piece configuration gasket construction is limited to relatively small diameters and construction of such seals of suitable material for cryogenic use is even more limited. Manufacturing capability of large diameter (up to approximately 20 feet) one-piece gaskets of suitable material is nonexistent.
Segmented gaskets with radial joints consist of a number of short segments of gasket material typically with interlocking ends. The pieces fit together to form a large gasket. These types of gaskets are ineffective because of short leak paths at the radial joints.
Construction of present spiral gaskets is limited to relatively small diameters. Typically, unlike the subject invention, they are not single component configurations. Their design requires many parts, complicated machinery for fabrication and a complicated technique for assembly. The construction of spiral wound seals in the prior art consists of inner and outer metal rings along with an intermediate metal spiral strip separating the gasket material in the spiral.
The paraphernalia used to retain the seal material is weighty and sizeable therefore eliminating its use as a practical alternative in some applications. Specifically, this construction prohibits its use in wind tunnel applications because access must be as large as the seal itself and as previously pointed out most wind tunnels do not provide such access. Also, large diameter seals of this construction are ineffective because differences in thermal expansion coefficients and other physical characteristics between the materials induce and accelerate degradation of the seal integrity when the seal is subjected to the severe wind tunnel environment. In the present invention the skived strip of material forms the total gasket. There are no intermediate strips or rings to affect seal performance.
The present invention consists of a single material strip, spiral wound into a groove in one of the sealed surfaces. The strip is wider than the groove is deep and thus protrudes from the surface. The opposing surface contacts and compresses the protruding strip portion to form a seal.
One known type of seal uses a single piece of material cut in the axial direction to form a spiral cut gasket, but this seal is used in a packing box around a shaft and is necessarily subjected to extreme compression from a gland to force the gasket material against the shaft to effect a seal to preclude axial leakage along the shaft.
The strip seal eliminates several design weaknesses of large diameter cryogenic pressure seals. This invention eliminates the necessity of having to fabricate a gasket to its final diameter at the manufacturer and also eliminates the accompanying shipping and storage problems associated with large gaskets. Further, because the gasket is of such a small size before installation, access to the seal location can be small compared to the seal final diameter thus eliminating much overdesign and costs. Therefore, this invention can be utilized at seal sites inaccessible to that of the prior art requiring complete gasket assembly during manufacture.
Inherent in the design of the spiral wound strip seal is the elimination of radial joints of the segmented gasket. In a seal utilizing a segmented gasket with radial joints the potential leak path is directly across the seal. The potential leak path in the strip seal is between the winds of the spirally wound strip and has a length equal to several times the circumference. Thus, in equal circumstances the chance for leakage of the spiral wound strip seal is only a minute fraction of the chance for leakage of a segmented radial joint seal.
Also, directly associated with the failure of seals using segmented gaskets in cryogenic applications is the shrinkage that occurs during the approximately 400.degree. F. temperature reduction. In segmented seals this opens a gap between the segments allowing leakage. In the proposed strip seal the thermal shrinkage tends to tighten the successive winds of the gasket strip upon themselves in the groove thereby enhancing the sealing capability.
The cost of the single strip seal is only a fraction of the cost of the methods used in the prior art. The seal gasket is composed of one component--the gasket material itself.
All intermediate strips, retaining rings, and mandrels for assembly are eliminated. The gasket is skived from a blank on a conventional machine lathe utilizing the two basic functions of the machine, namely; turning and crossfeed. During manufacture there are no additional machine settings or measurements required. There is no material wasted during fabrication. The finished seal is the same size as the original material blank.
Thick one-piece and segmented seals are inherently subject to warpage and damaging stresses. Using the spiral strip concept, a thick seal can be built up without warping because the individual strips are thin and the seal configuration tends to relieve the stresses.
Cutting a cross section through the seal shows the individual winds as columns which bend under a compression load. This configuration conforms to a warped surface better than a solid cross section as is in a one-piece seal or a segmented seal and improves sealing capability at low clamping pressures.
It is an object of the present invention to provide a seal utilizing a single strip spiral wound gasket.
It is another object of the present invention to provide a single strip spiral wound gasket wherein the strip is wound into a groove cut into one of the sealed surfaces.
It is a further object of the present invention to provide a seal utilizing a low cost, easily manufactured spiral wound gasket capable of maintaining seal integrity through severe pressure and temperature gradients.
A further object of the invention is a seal having the foregoing advantages and which holds under either low or high compressive forces.
Another object of the invention is a spiral wound seal with the foregoing advantages wherein a thin strip is wound to form a thick gasket not subject to warpage and damage stresses inherent in one-piece seals.
Other objects and advantages of the present invention will be readily apparent from the following description and drawings which illustrate the preferred embodiments of the invention.